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For in-process gpu, the CommandLine of the current process is a shared
resource and cannot be safely manipulated on the IO thread.

This moves inserting the gpu command line switches to the UI thread, and
slightly start up. Although still need additional fixes to start up code
to ensure it's safe to manipulate it on UI thread.

This also fixes the bug of inserting gpu switches when GpuProcessHost is
not used, for example in android webview.

BUG=450396

Review URL: https://codereview.chromium.org/865063002

Cr-Commit-Position: refs/heads/master@{#313177}